linux-ext4 - Re: 64bit filesystem questions

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Message-Id: <692307AB-41C8-4BC7-9D01-E5798CAB3548@dilger.ca>
Date:	Thu, 9 Jun 2011 18:08:24 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	Phillip Susi <psusi@....rr.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: 64bit filesystem questions

On 2011-06-09, at 8:36 AM, Phillip Susi wrote:
> I was checking up on the support for 64bit ( > 16 TB ) fs support in the next branch last night and have a few questions:
> 
> 1)  Why is blocks_per_group limited to 32k ( more specifically, 8 x blocksize )

There is only a single block pointer for each bitmap per group.  That said,
with flex_bg this is mostly meaningless, since the bitmaps do not have to
be located in the group, and a flex group is the same as a virtual group
that is {flex_bg_factor} times as large.

> 2)  Why can't 64bit be enabled explicitly ( with -O or -E? ) instead of automatically when needed and the enable automatic 64bit setting is in mke2fs.conf?

I thought it could be enabled with "-O 64bit", but I admit I've never tried.

> 3)  Why does 64bit disable the resize inode?

Because the on-disk format of the resize inode is only suitable for 32-bit
filesystems (it is an indirect-block mapped file and cannot reserve blocks
beyond 2^32).  The "future" way to resize filesystems is using the META_BG
feature, but the ability to use it has not been integrated into the kernel
or e2fsprogs yet.

Cheers, Andreas

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